Recently, we have developed an in vivo cloning and gene modification system using lambda-mediated homologous recombination (Red) with short (<50 bp) homologies. This system eliminates the need to cut DNA with restriction enzymes or join DNA fragments with DNA ligase. This extremely efficient recombination system could, we believe, revolutionize the way in which recombinant DNA work is done. Both ssDNA oligos and dsDNA oligos as short as 40bases can be used to generate recombinants in the bacterial chromosome or on genomic BAC library clones.